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Bt pesticide resistance
On Tue, 12 Aug 2003 07:37:17 -0700, Walter Epp
posted: "Moosh:}" wrote: On Thu, 07 Aug 2003 07:20:22 -0700, Walter Epp posted: "Moosh:]" wrote: On 29 Jul 2003 08:52:24 GMT, Brian Sandle posted: As we discussed with DDT, anything used for too long breeds resistant creatures. So? The point is that the use of BT in the plant and on the plant is hardly different. When the insects are not present, they can't be developing resistance. Where is there a place without insects? The relevant insects are those that damage the crop. If they don't, they won't be ingesting BT. but they can pass resistance genes to those who didn't ingest but can fly in and have a resistant feast. Only if they breed with the resistant ones and have resistant offspring, but this happens all the time. Welcome to the real world, where things are not black and white, where we don't have either 0 or trillions of insects but varying degrees inbetween, where not all insects are dumb enough to keep eating bt until they've got a fatal dose but different ones eat different amounts and so trigger varying amounts of selective pressure. And this happens with applied BT, only better coz the BT slowly reduces due to washing off and so on. So if you want to be accurate, applied BT can be worse than expressed BT wrt resistance development. Applied Bt is the most accurate way to minimize selective pressure. The crude approach of continual and high exposure makes for high selection pressure for resistance. No, you apply everytime you have pest damage. That application wanes. If no pest damage, then there is no contact wih the expressed BT. The bottom line is it makes no difference in the end. Just get used to the fact that pesticides will lose their effect sooner or later, and new ones must be developed. The old ones may be returned to at a later date, and different strategies can be used to minimise resistance formation. Resistance 0ccurs whenever a pest is partially killed by a pesticide. This can happen with applied or expresssed BT at more or less the same rate. When the pesticide is interrupted then resistance to it is no longer an advantage. And the pest destroys your crop, and you go bankrupt. Not necessarily, if the natural predators have not been wiped out by overuse of pesticides and the plants natural defenses have not been weakened by toxic and/or cultural damage to the soil ecology. BT is very specific, so your fear of pest predator damage is unfounded. Why are you postulating that the natural defences of the plant will be weakened? What are you trying to say about the soil ecology? Mycorrhizal fungi can effectively connect their plant hosts with as much as 1,000 times more soil area than the roots themselves. A single gram of soil may contain several miles of fungal hyphae. As they pump water and mineral nutrients to the roots, the fungi form a protective armor against disease bacteria around the roots, and sometimes innoculate the soil with antibiotics that kill disease bacteria. Root zone fungi and bacteria exude glues (polysaccharides) that bind soil particles together, resulting in better retention and movement of air and water. Mycorrhizal fungi break down nitrogen into forms that can be used by plants. Mats of fungi in the soil store nutrients that otherwise would be likely to dissolve and leach away. Roughly speaking. Roundup/Glyphosate is toxic to many beneficial mycorrhizal fungi, inhibiting growth at levels as low as 1ppm, and increases susceptibility of crop plants to a number of diseases. In vitro, I believe. Glyphosate will not reach the majority of the roots of fungal hyphae in real soils. It is too strongly bound to surface soil particles. Now the wetting agents may be a different matter. Dish liquid/hair shampoo is what caused the problems with amphibia. The mycorrhizal hyphal network is easily disrupted by mechanical disturbance. Disking a field, for example, can greatly reduce the ability of the soil to make new plants mycorrhizal, even though no fungal material is actually removed by disking. Exactly why "no till" using Roundup is so much better in so many places.. Then DDT will work again, or Bt. But if it is there all the time resistance to it remains an advantage for pests. Sorry, "there all the time" means nothing if the pests are not there. It might as well be withdrawn if the pests are absent. No contact, no advantage for the resistant mutations. When home gardners use it, or non-GM soy farmers &c, it is only present as needed, then disappears. And why does it matter if it's there or not, if the pests aren't predating the crop? There are always a few about, from the mandatory refuges, or other crops near by. But how does this matter? The chances of a resistance mutation are so much lower. Check out what's already happened: Independent on Sunday (London) March 30, 2003 INSECTS THRIVE ON GM 'PEST-KILLING' CROPS BY GEOFFREY LEAN ENVIRONMENT EDITOR Genetically modified crops specially engineered to kill pests in fact nourish them, startling new research has revealed. Biotech companies have added genes from a naturally occurring poison, Bacillus thuringiensis (Bt), which is widely used as a pesticide by organic farmers. Drawbacks have already emerged, with pests becoming resistant to the toxin. Environmentalists say that resistance develops all the faster because the insects are constantly exposed to it in the plants, rather than being subject to occasional spraying. Occasional spraying will result in many occasions where dose is sublethal. Ideal circumstances for resistance development. If the spraying is only occasional the selection pressure is low. Rubbish. If you kill half the pests occasionally, allowing the resistance gene to multiply and strentgen, you are going to get much more resistance problem. Keep up the 'cide constantly, and you kill many more pests. If the exposure is continual and high the selection pressure for resistance is high. The exposure is only continual when the pest are doing damage, so you would be applying continually anyway. There is very little difference. |
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